Vintage Pen - Dunn-Pen 1920 Writing Instrument

This item is marked Dunn-Pen , New York , Patented Nov. 23, 1920. The cap is in good condition and not split. The nib is inscribed Dunn-Pen Camel and appears to be in good condition (I am not an authority). The end of the pen has a threaded area w a "cap" of some sort once went, it is missing. T is a tube visible inside the pen. Sorry I don't know any more about it than that. Thank you for looking.